Depending on the target application, it is known to control a switched mode converter such as a flyback converter using different control modes, each of which has different advantages and disadvantages:
In continuous conduction mode (CCM), the magnetising current of the magnetic component, such as the transformer of a flyback converter, increases from a non-zero and positive minimum level to a maximum level during a primary stroke, then it decreases back to the minimum level during the secondary stroke. CCM provides the lowest RMS (root mean square) losses both on the primary and secondary sides. However the switching losses are high, particularly the capacitive loss associated with switching on at a voltage of Vin+N*Vout in case of a flyback converter. Also “soft switching” or zero current switching (ZCS) is not available, and there are further losses due to the reverse recovery of the output diode. In low load conditions the efficiency falls off rapidly, since the capacitive switch-on losses remain constant.
In boundary conduction mode (BCM) the magnetising current of the magnetic component such as a transformer increases from effectively zero to a maximum level during a primary stroke, then it decreases back to zero during the secondary stroke. The secondary stroke ends when the current returns to zero, and any subsequent primary stroke immediately follows. Thus current does fall to zero, but there is no gap in the conduction: the conduction mode is thus at the ‘boundary’ between being continuous and discontinuous. In practice, in BCM, the voltage across the switch usually drops at the end of the secondary stroke, since a ringing or resonance is set up when the current stops; the switching is usually effected at the bottom of the first valley in the voltage across the switch. However, in a variant of BCM known as BCM with valley skipping, the switch-on may be delayed by one or more cycles of this ringing—that is to say, one or more valleys are ‘skipped’, before the primary stroke is restarted at a further ‘valley switching’ moment.
BCM mode (with or without valley skipping) has the advantages of low switching losses, (no reverse recovery losses and low capacitive turn-on losses due to valley switching in particular), and a low value of the primary inductance of the magnetic component, enabling a relatively low number of turns, with consequentially reduced series resistance; however the switching frequency varies with input voltage and with load, and due to the principle of BCM operation the switching frequency increases significantly with decreasing loads, giving high switching losses at very high frequencies corresponding to very low loads.
Discontinuous conduction mode (DCM) benefits from lower inductance values. Switching frequency is typically fixed. However, at low load idle time is relatively large resulting in considerably higher peak current and high RMS losses, relative to BCM operation; also, valley switching is not available so that the switching losses are higher than under BCM.
Controllers are known which combine BCM and DCM. Here, at full power true BCM is operated with valley switching; if the load is decreased the switching frequency increases until an upper limit is reached known as a frequency clamp. Thereafter, that is to say at still lower loads, either DCM is applied, or BCM is continued but with valley skipping. This combination of modes suffers from a disadvantage at high load conditions with low input voltage, since then the peak current becomes very high (the switching frequency is reduced, and because, e.g. in the case of a flyback the amount of energy transferred per switching cycle is ½LI2f (where L corresponds to the magnetizing inductance, I is the maximum magnetizing current, and f is the switching frequency), the current goes up with lowering frequency). The transformer must be designed for this maximum load at minimum input voltage: the transformer is then suboptimal for intermediate and low loads. In order to accommodate with the inductance value, a high number of turns with at the same time a large air gap has to be used to prevent saturation. Also the RMS current through the switch becomes very large.
Controllers are also known which combined CCM and DCM. The switching frequency is typically fixed. This requires a complex control strategy since CCM has a two pole open loop response, and may have a right half plane zero, while DCM has a simple single pole response.
A datasheet for a power switch, FSQ510, from Fairchild, discloses a control method including a CCM at a high load, and a combination of BCM and BCM with valley switching at intermediate and low loads. However, this solution is not ideal since it requires a very high switching frequency at a relatively low partial load; moreover, it would be nontrivial to fix this disadvantage, since the control strategy uses of the lower frequency limit for the change from CCM to either BCM or DCM, and the lower frequency limit is directly tied to the upper frequency limit, which is itself fixed.
It would be desirable to provide a method of controlling a converter such as a fly-back converter which benefits from at least one of a flexibility of control mode, a relatively simple control strategy, and relatively low losses for a relatively wide voltage and or load range.